During the first cycle of the grant we explored the mechanisms underlying the anti-atherogenic effects of apolipoprotein E (apoE) expressed by macrophages, and delineated a unique hepatic axis between LDL receptor (LDLR) related protein (LRP) and apoE. Because both LRP and apoE are abundantly expressed in the macrophage, we postulate that this axis is operational in the vessel wall as well, where it may direct the uptake of intimal lipoproteins to a specific intracellular routing. Specific aim 1 will address the role of macrophage LRP in atherogenesis. The hypothesis tested is that LRP is the mediator of the anti-atherogenic effects of apoE in the artery wall, and that its deletion will promote lesion growth. Because apoE is a physiologic driver of cholesterol efflux from cells, its anti-atherogenic effects may be mediated by a more complex regulation of cholesterol homeostasis involving both uptake and disposition of macrophage cholesterol. Specific aim 2 will address the effects of apoE receptor binding defective variants expressed by the macrophage on cholesterol efflux and lipoprotein uptake, as well as their interaction with macrophage LRP. The hypothesis tested is that apoE affects cholesterol efflux in vivo not only by acting as an accepter but also by simulating LRP-mediated lipoprotein uptake. Multiple pathways to cholesterol efflux are present in macrophages, and the ATP-binding cassette (ABC) transporters and the scavenger receptor type B1 (SR-B1) can act as channels that deliver cellular cholesterol to extracellular acceptors. ABCA1 transposes phospholipids and cholesterol to apoAI. SR-B1 is normally involved in hepatic HDL cholesterol uptake, but in the macrophage cholesterol can also flow in the opposite direction and result in net efflux. Specific aim 3 will study the mechanism of apoE-mediated cholesterol efflux from macrophages and its relationship, if any, with either ABCA1 or SR-B1. The hypothesis tested is that apoE-mediated cholesterol efflux from macrophages is independent from ABCA1 or SR-B1 mechanisms.